Shock Structure, Mode Shape, and Geometric Considerations for Low-Pressure Turbine Flutter Suppression

Abstract

Modern design principles are pushing for lighter aircraft engines, and higher tip Mach numbers in steam turbines. Consequently, the loading and shock structures seen on aft stages of low-pressure turbines are evolving at a fast pace. Since flutter is a high risk event in these low reduced frequency blades, physically understanding and predicting how new operating conditions affect aeroelastic behavior proves crucially important. Previous numerical studies on the Standard Configuration 4 LPT concluded loading, and in particular the shock structure, strongly impacted the aerodynamic damping as a function of mode shape. This paper presents a deeper investigation into the physical explanation behind the interdependency of loading, mode shape, reduced frequency, and LPT geometry. Serving as computational test rigs, four unique quasi-2D LPT configurations are exhaustively analyzed using validated linearized unsteady CFD. Three primary contributors to this mode-dependent flutter phenomenon are shown to be the passage shocks, blade geometry, and interblade phase angle. Further, the sensitivity of critical reduced frequency as functions of bending mode and loading are divided into four zonal regions exhibiting qualitatively similar behavior. It is shown in some zones, depending on airfoil geometry and shock strength, that loading will always either increase or decrease stability monotonically. For pitching modes, the shock strength, phase, and location with respect to the rigid body center of torsion prove to play a key part in determining stability. Understanding these shock effects generalized to all LPT geometries yield new design considerations useful for suppressing flutter.